The major thrust of this amended application continues to be a multidisciplinary approach to study the mechanisms that regulate metabolic activities and differentiating events in the developing central nervous system. During the past five years the research supported by this Program Project Grant has been instrumental in establishing that various neural cell types have markedly different specificities with regard to the substrates they utilize. Since alterations in the patterns of substrate use during development may cause abnormal neural function manifested as mental retardation, the proposed studies focus upon the biochemical development of specific types of neural cells, individually and in interaction with each other, as well as the regulatory mechanisms that control their utilization of substrates. Furthermore the blood brain barrier changes adramatically during early development and changes in this process could also result in major neurological damage. To understand these interrelationships requires a comprehensive delineation of regulatory mechanisms underlying neural development. Such mechanisms include the regulation of gene expression, substrate transport, as well as regulatory mechanisms involving precursor-product relationships, post-translational modifications of key proteins, intracellular translocation, and intercellular trafficking of metabolites. The four projects address one or more aspects of this research theme both as individual investigations and as collaborative efforts. The role of carrier mediated transport, changes in pool sizes of metabolites, and effects on specific enzyme activities in the regulation of the metabolic fates of ketone bodies, malate and lactate will be the emphasis of one project. A second project will determine the molecular mechanism underlying the expression of glutamine synthetase and glutaminase activities in specific brain cells and how the regulation of these enzymes affects compartmentation, metabolic trafficking and oxidative metabolism. A third project uses a novel monoclonal antibody as a probe to characterize the developing blood brain barrier. The fourth project will focus on the modulatory properties of the S1OO Beta protein in specific brain cells and on effectors which regulate choline acetyl transferase activity. The results from this integrated, multidisciplinary approach should enhance our understanding of the complex metabolic alterations underlying many types of mental retardation and they will also provide guidance for the clinical management of infants with developmental disabilities.